Synergistic application of melatonin and methyl jasmonate mitigates drought-induced oxidative and photosynthetic impairment in <i>Brassica napus</i>

Drought stress severely limits rapeseed (Brassica napus L.) growth and productivity by disrupting photosynthetic efficiency and inducing oxidative damage. This study investigated the potential of melatonin (MT) and methyl jasmonate (MJ), individually and in combination, to mitigate drought-induced toxicity in rapeseed. Plants were subjected to drought stress (15% Polyethylene glycol 6000), followed by foliar application of MT (100 μM), MJ (100 μM), or their combination. Under drought stress, rapeseed exhibited significantly reduced chlorophyll content (by 33.72% for chl a and 54.59% for chl b), impaired photosystem II efficiency (Fv/Fm decreased by 16.08%), and elevated oxidative stress markers (H₂O₂ increased by 54.79%, MDA by 144.6%, ROS by 43.96%). The combined application of MT and MJ conferred the highest level of drought tolerance, increasing shoot fresh weight by 92.33% and root dry weight by 405% compared to the stressed control. This synergistic treatment effectively enhanced stomatal conductance by 76.25%, increased net photosynthetic rate (Pn) by 71.45%, and boosted key antioxidant enzyme activities (e.g., POD by 216%). The enhanced efficacy of the dual treatment compared to individual applications strongly indicates coordinated signaling between MT and MJ pathways in stress response modulation. These results demonstrate that simultaneous application of these plant growth regulators offers an effective physiological strategy to improve rapeseed performance under water deficit conditions.